The team found that the dung of treated cows produced nearly twice as much methane – an important greenhouse gas – as that of non-treated ones.

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“We were surprised to find such a big increase in methane emissions in dung,” says Tobin Hammer of the University of Colorado in Boulder. “We believe that the tetracycline treatment favours the growth of methanogenic archaea in the cows’ intestinal tract by reducing the bacteria in the gut.”

Although these changes didn’t kill the beetles or prevent them from reproducing, Anne Lizé, a researcher at the University of Rennes 1 in France, says they could alter their behaviour and sense of smell, as well as the way they interact.

“The disruption of microbiota could lead to indirect behavioural effects that happen not only in the focus organism – the one that has been administered an antibiotic – but also in related communities living in or around it,” she says. These changes were not monitored in the study.

“I am surprised to see such a strong effect,” says Kevin Floate, researcher at Agriculture and Agri-Food Canada. “It would seem important to assess the effect of different types of antibiotics, which have different modes of action and affect different types of bacteria.”

Floate adds that it would be useful to look at how long after treatment antibiotics continue to be excreted at levels that affect these microbiomes.